4. At an effective annual interest rate 1', the present value of $20 per year, payable at the beginning of each year for ten years, is $135.20. At the same interest rate, the accumulated value at the end of ten years of a single payment now of $20 is $51.88. Find the accumulated value (still at this same interest rate) at the end of twenty years of $20 per year, payable at the end of each year